WAUSAU, Wis (WSAU) The cause of an underground explosion that sent manhole covers flying into the air on Stewart Avenue in Wausau is still a mystery. Investigators say failed insulation around some underground wires may be to blame.

City officials and WPS have traced the source of the explosion to the underground line between a transformer and the main service panel at the LaQuinta Inn. The heat build-up and other materials underground could have led to the pressure that caused the blast.

The same hotel was the scene of a natural gas build-up in 2011. It's not thought that the two incidents are related.

The sidewalk and alleyway outside the motel are closed while the investigation continues.